1. Field of the Invention
The present invention relates generally to a gas monitoring system, and more specifically to a system including a filter and a gas sensor, in which the filter removes hydrogen sulfide and/or chlorine and/or hydrogen chloride from a source gas that is provided to the gas sensor for sensing of other component(s) therein.
2. Description of the Related Art
Gas sensors are used in many applications for the detection of hazardous gas component(s) in a gas stream or gas environment. These hazardous gas component(s), hereafter referred to as “target gas,” may be of widely varying types. Their hazardous character may derive from their toxicity to humans, pyrophoricity, explosive character, flammability, deactivating character as regards materials used for abatement or reclamation of other components in the gas mixture.
In many applications, the gas sensor is not strictly selective for the target gas, and the other component(s) of the gas being monitored may interefere with or preclude the proper operation of the gas sensor. For example, gas component(s) other than the target gas can produce the same signal or response by the sensor, so that the concentration of the target gas in the gas stream or environment being monitored is misattributed by the gas sensor.
Such misattribution of the concentration of the target gas can have severe consequences for the process operation or action that is conducted based on the sensed concentration of the target gas. For example, vital steps of an industrial process may be curtailed or unduly prolonged due to the incorrect sensing of target gas, with consequent adverse effect on the process economics or safety. Action may be taken based on the misattributed target gas sensing that is wasteful or even superfluous.
Such undesirable behavior of the gas sensor can be prevented by the use of gas filters that remove from the gas being monitored by the gas sensor, those component(s) that would otherwise interfere with the accurate sensing of the target gas by the gas sensor.
The present invention relates to gas filters for such purpose, and to gas monitoring systems that comprise such filters.
Phosgene (COCl2) is a chemical of major industrial importance. The annual production worldwide is more than 1 million tons, 90% of which is used in the manufacture of isocyanates and polyurethane and polycarbonate resins. Phosgene is also extensively used as a synthetic reagent in a wide variety of organic chemical processes, e.g., the synthesis of numerous chloride compounds.
Phosgene also is a hazardous chemical compound, since it readily decomposes in the presence of water to yield HCl and CO2. Phosgene also is highly adsorbable, even by such chemically inert materials as polytetrafluoroethylene (PTFE), in addition to being highly toxic, irritating and corrosive in character. Inhalation of phosgene can cause fatal respiratory damage. Due to its colorless, odorless character, phosgene is a gas that requires, sensitive, accurate and reliable monitoring in gas streams or environments in which it is or may be present.
Due to its hazardous character, the maximum workplace concentration (MWC) of phosgene during a 40 hour week in a workplace environment is 0.1 parts per million by volume (ppmv).
Chlorine dioxide (ClO2) is another chlorine-containing hazardous gas, whose MWC value also is 0.1 ppmv. ClO2 is manufactured on a large scale, as is used as a substitute for chlorine or ozone in many industrial applications. Its uses include biocidal applications (e.g., in the pulp and paper industry), disinfection applications (in municipal water treatment, treatment of medical waste, and food applications), circuit board cleaning in the electronics industry, treatment of sulfides in the petroleum industry, and bleaching applications in the textile industry, to name a few. An advantage of using ClO2 is that it does not directly form halogenated byproducts, as is the case when chlorine is employed. Like chlorine, ClO2 is a very strong oxidant. ClO2 also has the advantage that it does not form dioxins.
ClO2, however, is not stable, and it therefore is typically produced at the point of use (POU) location, in the amount that is required. Chlorine dioxide is a highly reactive gas, readily entering into disproportion reactions, decomposing to HCl and HClO3 in the presence of water, or to ClO3 and H2O in alkaline solution. ClO2 is able to react as an oxidative or a reductive agent. It can be oxidized by strong oxidants such as potassium permanganate but in many instances reacts as an oxidant itself. Chlorine dioxide is highly adsorbable, e.g., by activated carbon. Due to its high toxicity, it is necessary to monitor chlorine dioxide in an accurate, sensitive and reliable manner.
Electrochemical sensors are widely used for measuring the concentration of toxic gases (see, for example, Advances in Electrochemistry and Electrochemical Engineering, Volume 10 (J. Wiley & Sons, 1976). A potential disadvantage of electrochemical sensors is their cross-sensitivity to other hazardous gases that may be present in the stream or environment being monitored for a target gas.
Considering the aforementioned gases COCl2 and ClO2 as target gas species, which are desirably monitored in environments and/or process streams containing same, it is to be noted that the presence of COCl2 and/or ClO2 gas in many applications is accompanied by the presence of hydrogen sulfide and/or chlorine and/or HCl. The latter gases are less toxic than phosgene or chlorine dioxide, as shown by their MWC values. Whereas COCl2 and ClO2 each have a MWC value of 0.1 ppmv, the MWC value of Cl2 is 1.0 ppmv, the MWC value of HCl is 5.0 ppmv and the MWC value of H2S is 10.0 ppmv.
H2S is easily oxidized in the following reaction:H2S31 +4H2O→H2SO4+8H++8e−and chlorine is a strong oxidant:Cl2+2e−→2Cl− Eo=1.36 volts
In electrochemical sensors for COCl2, phosgene produces an anode current. In electrochemical sensors for chlorine dioxide, the ClO2 gas produces a cathode current by the following reduction reactionClO2+4H++5e−→2H2O+Cl− Eo=1.27 volts
In such sensors for phosgene and chlorine dioxide, the sensor response to H2S has the same polarity as the sensor response to phosgene, and the opposite polarity to the response of the sensor to chlorine dioxide.
Thus, the presence of hydrogen sulfide in an air mixture with phosgene will produce a false higher response of the sensor to phosgene, and the presence of hydrogen sulfide in an air mixture with chlorine dioxide will produce a false lower response of the sensor to chlorine dioxide, even when the hydrogen sulfide in the respective air mixtures is at a level below the MWC value.
Correspondingly, in such sensors for phosgene and chlorine dioxide, the sensor response to chlorine has the opposite polarity to the response of the sensor to COCl2 and the same polarity as the response of the sensor to ClO2.
Thus, the presence of chlorine in an air mixture with phosgene will produce a false lower response of the sensor to phosgene, and the presence of chlorine in an air mixture with chlorine dioxide will produce a false higher response of the sensor to chlorine dioxide, even when the chlorine in the respective air mixtures is at a level below the MWC value.
When both hydrogen sulfide and chlorine are present with the target gas in a three-component gas mixture, the phosgene sensor or chlorine dioxide sensor will show a superpositional response, i.e., an algebraic summation of the responses of the sensor to each gas component.
Hydrogen chloride (HCl) poisons COCl2 sensors, which typically use gold working electrodes. It is thought that the Cl− anion forms complexes with the gold electrode thereby preventing accurate determination of ClO2 concentration. As one example, 10.0 ppmv HCl distorts a ClO2 sensor signal by between about 150 and 300 nA.
Hydrogen sulfide, chlorine and hydrogen chloride are also interferent gas components for other electrochemical gas sensors, e.g., those employed for monitoring of target gas species such as sulfur dioxide, nitrogen dioxide, hydrogen, hydrogen chloride and ammonia.
The use of chemically selective filters is known in the art, wherein the filter effects removal of the interferent gas species from the gas being monitored, so that the filtered gas subsequently exposed to the gas sensor produces a concentration sensing for the target gas that is unaffected by the presence of the interferent gas species, and thereby accurate for the target gas. For example, hydrogen sulfide filters are described in Warburton et al. U.S. Pat. No. 6,284,545 and are otherwise known, which operate by oxidation or adsorption of the hydrogen sulfide component of the gas mixture containing same, using filters employing manganese dioxide, potassium permanganate, activated carbon, activated carbon with manganese dioxide, etc. Such filters are effective in removing hydrogen sulfide as well as chlorine, but at the same time they also remove phosgene and chlorine dioxide with very high effectiveness. In consequence, these filters produce a filtered gas that is misrepresentative of the concentration of phosgene and chlorine dioxide in the original source gas (i.e., prior to filtering), producing false lower sensed concentrations of the target gas. Such false low reading of the target gas concentration by the gas sensor thus creates a situation of potential danger to personnel in the vicinity of the source gas as well as inadequate treatment or processing of gas due to the false lower sensed concentration of the target gas.
The art therefore is in need of a gas sensing system for monitoring concentration of phosgene and chlorine dioxide in instances where the source gas being monitored contains hydrogen sulfide and/or chlorine, and/or hydrogen chloride.